A number of protection systems are known in the electrical discharging machining art in which electrical parameters of gap conditions are responded to when these are of an abnormal level to provide interruption of the power supply particularly in response to gap short circuit condition. The present invention is related to an embodiment of an EDM power supply in which the output switches are transistors, although it is equally applicable to any precision type EDM power supply in which electronic switches are used to provide machining power pulses to the machining gap. By "electronic switch" we mean any electronic control device having a plurality of electrodes including at least two principal or power conducting electrodes acting to control current from the power circuit, the conductivity of the power circuit being controlled by a control electrode within the switch so that the conductivity of the power circuit is statically or electrically without the movement of any mechanical elements within the switch. Included within the definition, by way of illustration and not limitation, are electronic tubes, transistors, semi-conductor control rectifiers, and the like.
The protection system of the present invention is specially adapted for use in small hole electrode EDM in which one or more wire-type electrodes are used to provide small slotted or channel openings in a workpiece. When the electrodes are very small, it is generally necessary to limit the current flow to four or five amperes. When this level is exceeded, the electrodes become heated and actually become white-hot. At this point, the usual cutoff circuitry used is unable to respond due to the fact that the reason voltage increased across the small electrode because it got hot and the resistance increased. The normal cutoff system which responds to sudden drop in gap voltage will not operate here but will continue to allow full current flow to the electrodes which will, in many cases, be burned up.
The essential problem is to provide a current limiting system for this type of EDM operation.
There are available a number of cutoff systems which interrupt the operation of a power supply responsive to an appropriately sensed abnormal condition. In some cases, responsive to abnormal gap condition, relays are operated which interrupt the drive pulses and thus immediately drop the amperage to the electrodes. This will normally place the circuit into and out of operation each time the amperage exceeds the predetermined level. One patent disclosing this type of cutoff system is shown and described in Bell, et al, U.S. Pat. No. 4,134,000, issued on Jan. 9, 1979, for "Protection Circuit for Electrical Discharge Machining Power Supply Apparatus".
Other protection systems for EDM are known which have a cutoff switch that operates responsive to gap short circuit condition to provide a cutoff or pulse narrowing signal to the multivibrator. In this way, triggering pulses to the output switch and thus machining power pulses to the gap are interrupted when abnormal gap condition arises. One system of this type is shown and described in Kurt H. Sennowitz, U.S. Pat. No. 3,590,317, issued on June 29, 1971 for "Electrical Discharge Machining Pulse Current Control Apparatus".
The inventive current limiting system according to the present invention may incorporate a variety of known cutoff circuits of the type referred to above in accomplishing that function.